India is home to the third largest population of HIV-infected persons globally and also to ~ 1.1 million injection drug users (IDUs). Data from our group and others show poor penetrance of voluntary counseling and testing (VCT) and risk-reduction services among IDUs, as well as late HIV diagnosis, poor uptake of antiretroviral therapy (ART), and high mortality among HIV-infected IDUs. A multi-level strategy - which facilitates access through an integrated structural intervention, disseminates the intervention by leveraging social networks, and addresses individual barriers (e.g., substance abuse, comorbidities) - is needed to improve HIV prevention and treatment among IDUs. Prior work from our group and others suggests that integrated models of care can improve outcomes in populations with comorbid substance abuse and medical conditions. We propose that IDU-oriented integrated care clinics (ICCs) can engage vulnerable IDU populations and improve community-level outcomes along the seek, test, treat and retain (STTR) continuum. Aim 1: Determine HIV prevalence and characterize access to preventive and treatment services among IDUs in 15 geographically distinct communities in India using ethnographic approaches and respondent-driven sampling (RDS). RDS is a chain-referral sampling method that is implemented in a manner that enables unbiased prevalence estimates of selected factors in the target population. We will use this information methodologically and analytically in Aim 2. Aim 2: Evaluate the effectiveness of IDU-oriented integrated care clinics (ICCs) for improving outcomes along the seek, test, treat, and retain (STTR) continuum using a cluster-randomized trial. We will conduct a cluster-randomized trial among 5 pairs of matched communities (10 communities total) selected from the 15 candidate communities assessed in Aim 1. Communities will be matched according to geographic region and proportion reporting VCT in the prior 12 months at baseline. Starting with existing IDU-oriented service providers, we will establish ICCs in the 5 intervention communities. ICCs will provide the 9 IDU services recommended in the World Health Organization (WHO) 'comprehensive package': HIV VCT; condom distribution; counseling and education; needle exchange programs; opioid substitution therapy; management of sexually transmitted infections, tuberculosis, and viral hepatitis; and ART. Community-level outcomes will be assessed with a second (evaluation) RDS conducted 24 months after ICC implementation. We hypothesize that establishing IDU-oriented ICCs will lead to increased access to VCT, reduced HIV transmission risk behaviors, and (among HIV-infected persons) increased access to clinical care, increased use of ART, and decreased community viral load. While our proposed study will be conducted in India, its results may be applicable to IDU-prevalent areas worldwide as fragmented delivery of these services is the norm in both developed and developing countries.